1. Field of the Invention
The present invention relates to an error control method and apparatus for wireless data communication, particularly to an error control method and apparatus for wireless data transmission over a digital mobile communication system such as a PHS (Personal Handy phone System), various digital mobile/car phone systems, or a digital private wireless LAN (Local Area Network) system.
2. Description of the Related Art
Recently, data transmissions via mobile terminals of a mobile communication system have been popular in practice with the spread of personal computer communications and internet access. These wireless data communications have been utilized not only for private use, but also for commercial use.
In general, in the radio link or radio frequency communications environment, transmission path conditions will greatly vary due to the special characteristics of radio propagation, such as fading. Therefore, most of the digital mobile communication systems adopt at least one error control strategy such as, for example, the FEC (Forward Error Correction) strategy, or the ARQ (Automatic Repeat Request) strategy, so as to reduce bit errors which may occur during passage through the radio link.
In the PDC (Personal Digital Cellular) system, which is one of the Japanese digital mobile phone systems, the data communication of 2400 bit/s adopts the FEC strategy using BCH (Bose Chaudhuri Hocquenghem) code and the data communication of 9600 bit/s adopts the ARQ strategy with a combination of Go-Back-N and Selective Repeat protocols. In other mobile communication systems, the FEC strategy, the ARQ strategy or the FEC/ARQ hybrid strategy may be adopted for data communication.
According to the conventional data transmission over mobile communication systems, the error control strategy is not changed and its parameters are also fixed while the communication link is kept in connection. For example, in the above-mentioned data transmission of 2400 bit/s over the PDC system, the BCH code length is fixed to 15 bits, the information length is fixed to 4 bits, and the depth of the associated interleaving is fixed to 73 while the connection is kept. Also, in 9600 bit/s data communication, both the frame length and the modulo number for ARQ are fixed to certain values during the connection.
However, since the transmission path conditions will always vary in the mobile radio link environment, the unchanged error control strategy and fixed parameters thereof will introduce a number of problems, as will be discussed below.
The error control strategy and its parameters are usually designed so that bit errors can be reduced even when the transmission path conditions are somewhat poor. Therefore, when the conditions are relatively good, it is not necessary to execute such error control. In other words, no bit errors will occur without executing such error control. In such conditions, it is preferable to increase the amount of information which can be transmitted per unit of time rather than executing such error control. For example, in the above-mentioned data transmission of 2400 bit/s over the PDC system, the maximum transmission efficiency can be obtained if all bits are used for data transmission under good transmission path conditions. However, only 4 bits in the 15 bits of the BCH code can be utilized for data transmission causing a very low transmission efficiency of 4/15. Similar to this, in the data transmission of 9600 bit/s over the PDC system, 48 bits are utilized for the ARQ control information whereas 176 bits are used for data transmission, also causing a low transmission efficiency of 176/224.
Conversely, when the transmission path conditions become worse than the conditions estimated in designing the error control method and its parameters, high transmission performance cannot be maintained by means of the designed error control method and the parameters. As a result, bit errors may occur in the data transmitted to the user, the transmission delay may exceed the maximum permissible limit, or the throughput may be lowered. In an extreme case, no data transmission is possible, causing the transmission link to be disconnected.